Electrically conducting polymers

ABSTRACT

The polymers resulting from reaction of a 1,1,2,2-tetrahaloethane with a diamine of the type H 2  NANH 2 , where A is a divalent aromatic moiety, may be doped with a variety of materials to afford electrically conducting polymeric compositions. Poly(ethyleneiminobenzene) is an especially useful material which may be prepared in good yield from the reactants in a dipolar aprotic solvent in the presence of a base.

CROSS REFERENCE TO A RELATED APPLICATION

This application is a continuation-in-part of my copending application,Ser. No. 433,943, filed Oct. 12, 1982, now U.S. Pat. No. 4,466,911, allof which is hereby incorporated within.

BACKGROUND OF THE INVENTION

Although organic polymers have replaced metals in many structuralcontexts, thus far they have failed to replace metals when the latterare used as electrical conductors or semiconductors. The impetus forsuch replacement includes, among others, lower cost, lower weight ofmaterials, and increased processing variability for polymers as comparedwith metals. For example, polymers readily can be cast as films, foils,and fibers by standard, time-tested procedures. Polymers can be formedinto a limitless variety of shapes and dimensions by standard processingprocedures, thereby adding to the potential benefit of electricallyconducting polymers.

A potential use for electrically conducting polymers is as electrodes orcomponents of batteries, where their low weight and possibly unlimitedscope of design are attractive. Electrically conducting polymers alsocould find use in construction of solar cells. Where such polymers arephotoconducting they would undoubtedly find applications in theelectrophotographic industry.

The conductivity ranges characterizing insulators, semiconductors, andmetallic conductors are somewhat arbitrary, but for convenience we maysay an insulator has a conductivity less than about 10⁻¹⁰ ohm⁻¹ cm⁻¹, aconducting metal has a conductivity greater than about 10² ohm⁼¹ cm⁻¹,and a semiconductor has an intermediate conductivity. In some casesorganic polymers which are insulators show a sufficient increase inconductivity upon doping to act as semiconductors. By "doping" is meantadding a compound, referred to as a dopant, to the polymer so as to forma redox system wherein an electron is transferred from the polymer tothe dopant, or vice versa. Two common examples of dopants are iodine (anelectron acceptor) and sodium naphthalide (an electron donor). When thepolymer transfers an electron to the dopant to exhibit semiconductorproperties it is called a p-type semiconductor because conduction occursmainly via holes in the valence band. Conversely, when the polymeraccepts an electron from the dopant to exhibit semiconductor propertiesit is called an n-type semiconductor because conduction occurs mainlyvia electrons in the conduction band.

It is desirable for a normally insulating polymer to become asemiconductor upon doping by both p- and n-type dopants. It is alsodesirable that the polymer respond to a wide variety of dopants, and forits conductivity to be relatively responsive to changed levels ofdopant. It is also desirable that the conductivity properties of thepolymer remain stable over time and upon air exposure of the polymer. Itis also quite desirable that upon doping the polymer remain flexiblerather than becoming brittle.

It is a discovery of this invention that certain condensation polymersof 1,1,2,2-tetrahaloethanes and aromatic diamines, and which have thestructure,

    (═CH--CH═N--A--N═).sub.x

where A is an aromatic moiety selected from the group consisting ofbenzene, naphthalene, biphenyl, pyridine, and acridine, are polymerswhich show many of the aforementioned properties. In particular, thepolymer from a 1,1,2,2-tetrahaloethane and 1,4-diaminobenzene,poly(ethyleneiminobenzene), hereafter referred to as EIB, is normally aninsulator whose conductivity increases to about 10⁻⁴ ohm⁻¹ cm⁻¹ upondoping with an electron acceptor such as iodine. In addition to theseproperties as a p-type semiconductor, EIB can be doped with an electrondonor such as sodium naphthalide to behave as an n-type semiconductor.An unexpected and highly advantageous property of EIB after being dopedis that some such doped polymers are pliable, in contrast to the brittlecharacter of undoped EIB, thereby facilitating production of shapedelectrically conducting polymers.

Polyacetylene and poly(p-phenylene) exemplify some better, perhaps thebest, prior art electrically conducting polymers, hence theirlimitations exemplify the prior art constraints. Although polyacetylenemay be doped with p- and n-type dopants, all doped as well as undopedpolyacetylene are unstable in air. Thus the electrical properties, whichare of greatest interest in this application, are useful for only shortperiods in air. In contrast, poly(p-phenylene) itself is air stable butit affords air unstable, electrically conducting polymers with both p-and n-type dopants. Moreover, these materials invariably are amorphouspowders which cannot be cast, hence their processability is severelylimited.

DESCRIPTION OF THE INVENTION

In one aspect the invention which is disclosed and claimed is a methodof preparing an electrically conducting polymer comprising contacting apolymer with the structure,

    (═CH--CH═N--A--N═).sub.x

with a dopant, and recovering the resulting doped polymer.

In another aspect the invention is the electrically conducting polymeritself, which comprises a polymer with the aforementioned structure anda dopant, the latter being present in a mole ratio up to about 4.

The polymers of this invention have an extended, linear conjugatedsystem which is believed to afford the basis of electron flow. In itsground state the polymer has its highest bonding orbital completelyfilled, and its lowest antibonding orbital totally unoccupied. In such astate the polymer would be nonconducting,--i.e., an insulator--becauseof the absence of charge carriers, despite the fact that both orbitalsabove are extensively delocalized. If the gap between the aforementionedorbitals were sufficiently small, thermal excitation would promote somefraction of electrons from the highest bonding orbital, or valence band,into the lowest antibonding orbital, or conduction band. There would be"holes" in the valence band and electrons in the conduction band, bothof which are charge carriers, and the polymer would be electricallyconducting. A similar electron transfer could accompany absorption oflight, in which case the polymer would be a photoconductor.

Because the band gap of the polymer in question is too large forappreciable thermal population of the conduction band, a different modeof introducing charge carriers is needed. As stated above, dopants arematerials which form a redox system with the subject polymer. Thefunction of a dopant is to introduce charge carriers into the polymer.Where the dopant is an oxidizing agent and removes an electron from thehighest filled orbital of the polymer it creates "holes" in the valenceband which are p-type charge carriers. Analogously, where the dopant isa reducing agent and adds an electron to the lowest unoccupied orbitalof the polymer it puts electrons into the conduction band which aren-type charge carriers. Dopants which cause a polymer to behave as p- orn-type conductors, respectively, are called p- or n-type dopants.

The polymers of this invention have the repeating unit,

    (═CH--CH═N--A--N═)

The moiety A is an aromatic divalent moiety whose parent is a moietysuch as benzene, naphthalene, anthracene, biphenyl, pyridine,azobenzene, acridine, stilbene, and so forth. Examples of aromaticdiamines which are the source of the moiety A include1,4-diaminobenzene, 1,3-diaminobenzene, 1,2-diaminobenzene,4,4'-diaminodiphenyl and the other isomeric diaminodiphenyls where eachof the rings bears only one amino group, the isomeric diaminopyridines,the isomeric diaminoacridines where each of the lateral fused ringsbears one amino group, and especially 3,6-diaminoacridine, the isomericdiaminostilbenes, particularly 4,4'-diaminostilbene, and the isomericdiaminoaphthalenes, diaminoanthracenes, and diaminoazobenzenes.

The polymers of this invention may be prepared by reacting in solutionapproximately equimolar proportions of a 1,1,2,2-tetrahaloethane andaromatic diamine in a dipolar aprotic solvent, such asdimethylacetamide, in the presence of a base in an amount sufficient toremove formed acid and at a temperature from about 20° C. to about 100°C. The use of a dipolar aprotic solvent permits the polymer to bereadily cast from such solutions to afford films which, once dry, areinsoluble. Additional examples of dipolar, aprotic solvents which may beused in the practice of this invention include dimethylsulfoxide,hexamethylphosphoramide, dimethylformamide, sulfolane, tetramethylenesulfoxide, nitromethane, and acetonitrile. Tertiary amines areconvenient and effective bases in the practice of this invention.1,1,2,2-Tetrabromoethane is the reactant of choice, although the chloroand iodo analogues may be used but not necessarily with equivalentresults.

The polymer EIB itself is an insulator, showing a conductivity less than10⁻¹⁰ ohm⁻¹ cm⁻¹. However, when doped with either p-type or n-typedopants the conductivity increases to about 10⁻⁴ ohm⁻¹ cm⁻¹. Where thedopant is itself air stable, it has been found that the electronicallyconducting doped polymer is relatively air stable as manifested byunchanged conductivity over a period of several weeks.

As examples of p-type dopants there may be a cited bromine, chlorine,and iodine; SbF₅, AsF₅, PF₅, and related compounds where fluorine isreplaced by a highly electronegative, non-nucleophilic group; AgClO₄ asrepresentative of a silver compound with a non-nucleophilic anion; NO₂ Xand NOX, where X is a non-nucleophilic anion such as SbF₆ ⁻, FB₄ ⁻, andso on; and sulfuric and perchloric acids.

Examples of n-type dopants are more limited. The most common suchdopants are alkali metal salts of the radical anions of aromaticsystems, as exemplified by sodium naphthalide.

Electrochemical doping introduces additional variants. For example, ifthe polymer serves as the cathode in a medium containing atetraalkylammonium perchlorate as the electrolyte, the polymer becomesreduced, thereby exhibiting n-type conduction, and thetetraalkylammonium cation serves as a dopant by impregnating the polymerto preserve electrical neutrality. When the polymer serves as the anodeit becomes oxidized and exhibits p-type conduction with the perchlorateanion serving as the dopant.

The amount of dopant incorporated into the polymer to give theelectrically conducting system is subject to wide variation, depending,for example, on the dopant and the conductivity desired. In the case ofiodine, for example, the mole ratio dopant to repeating unit of polymermay be as low as about 0.5 and up to about 5. Generally a mole ratiofrom about 0.1 to about 3.5 is most commonly employed.

When EIB was treated with some of the dopants of this invention itdisplayed a remarkable change in physical properties. For example, whenEIB was doped with iodine the originally brittle material assumed apliable, somewhat tacky consistency. This iodine-doped polymer could bereadily "painted" onto surfaces, i.e., deposited as a thin film assumingthe shape of the surface, and could be readily molded into virtually anyshape desired.

The examples given below merely serve to illustrate this invention andare not intended to limit it in any way.

EXAMPLE

Amorphous EIB was prepared as follows. To a 100 ml 3-necked,round-bottomed flask equipped with a mechanical stirrer and a refluxcondenser were added, in a nitrogen atmosphere, 1.49 g (4.21 mmol) of1,1,2,2-tetrabromoethane and 15 ml of N,N-dimethylacetamide. To thestirred solution was added 0.46 g (4.26 mmol) 1,4-diaminobenzene. Thesolution was stirred 0.5 hr at ambient temperature and 24 hrs at 55° C.,whereupon 5 ml triethylamine was added. After 4 hr the volatiles wereremoved, in part, by a stream of nitrogen blown over the surface. Solidswere removed by filtration and the liquid was permitted to solidify byexposure to air with accompanying evaporation of volatiles. The polymerhad a glassy, dark purple appearance.

Pellets for electrical conductivity testing were prepared by adding ameasured amount of the polymer powder to a Beckman IR pellet press. Thepellets were 1.3 cm in diameter with a thickness determined by theamount of material pressed and the pressure used.

To obtain reliable conductivity data which could be duplicated it wasfound necessary to dry the material thoroughly in vacuum. Accordingly,after being pressed all pellets were dried at 100° C. at 0.2 mm Hg forseveral hours. Pellets normally were removed and stored under nitrogenuntil testing.

Iodine doping was carried out by adding an iodine crystal to a chambercontaining a pellet of EIB. The chamber then was evacuated causingimmediate sublimation of iodine. Gaseous iodine remained in contact witha pellet for a period from about 1.5 to about 17 hours, whereupon thedoped pellet was removed and stored under nitrogen until being tested.

For example, when a pellet of EIB, whose conductivity was 10⁻¹⁰ ohm⁻¹cm⁻¹, was exposed to iodine for 17 hours it incorporated 1.07 moleiodine per mole EIB, with the doped material having a conductivity of7.1×10⁻⁵ ohm⁻¹ cm⁻¹.

Doping of EIB with sodium naphthalide may be accomplished by contactingthe polymer powder with a slurry of sodium naphthalide in drytetrahydrofuran. After the mixture is stirred under nitrogen for 24hours, excess sodium naphthalide and solvent may be removed with apipette. The remaining solvent may be evaporated in a stream of nitrogenand the doped polymer may be dried as described above but at roomtemperature.

What is claimed is:
 1. A method of preparing an electrically conductingsolid polymeric composition comprising contacting a polymer whoserepeating unit has the structure,

    (═CH--CH═N--A--N═)

where A is an aromatic divalent moiety whose parent is selected from thegroup consisting of benzene, napthalene, anthracene, biphenyl,azobenzene, pyridine, acridine and stilbene with an effective amount,sufficient to increase electrical conductivity, of a p-type or n-typedopant and recovering the resulting doped polymer.
 2. The method ofclaim 1 where A is the 1,4-divalent benzene moiety.
 3. The method ofclaim 1 where the dopant is an n-type dopant.
 4. The method of claim 3where the dopant is an alkali metal salt of an aromatic radical anion.5. The method of claim 1 where the dopant is a p-type dopant.
 6. Themethod of claim 1 where the doped polymer is electrochemically doped. 7.The method of claim 5 where the dopant is selected from the groupconsisting of chlorine, bromine, iodine, SbF₅, AsF₅, PF₅, AgX, NO₂ X,and NOX where X is an unreactive, non-nucleophilic anion.
 8. The methodof claim 1 where an effective amount is a mole ratio of dopant topolymer up to about
 5. 9. A method of preparing a solid polymer with thestructure,

    (═CH--CH═N--A--N═).sub.x

where A is an aromatic divalent moiety whose parent is selected from thegroup consisting of benzene, naphthalene, anthracene, biphenyl,pyridine, acridine, and stilbene, comprising reacting in solutionapproximately equimolar proportions of a 1,1,2,2-tetrahaloethane and anaromatic diamine selected from the group consisting of diaminobenzene,diaminonaphthalene, diaminoanthracene, diaminodiphenyl,diaminoazobenzene, diaminopyridine, diaminoacridine, anddiaminostilbene, in a dipolar aprotic solvent in the presence of a basesufficient to remove formed acid and at a temperature from about 20° C.to about 100° C., and recovering the resulting polymer.
 10. The methodof claim 9 where the halo moiety is selected from the group consistingof bromine, chlorine, and iodine.
 11. The method of claim 10 where thehalo moiety is bromine.
 12. The method of claim 9 where the reactantsare 1,1,2,2-tetrabromoethane and 1,4-diaminobenzene.